1. Field of the Invention
The present invention relates generally to the field of lead acid batteries and, more particularly, to a battery grid structure having a grid lug. Still more specifically, the present invention relates positive and negative battery plates made from such grid structures. In a further refinement of the present invention, the invention relates to grid structures which facilitate current collection of the lug, especially for positive battery plates. This permits the elimination of individual battery plates, yielding lighter weight batteries or similar weight having dramatically improved performance characteristics.
2. Description of Prior Battery Systems
Lead acid batteries typically comprise a series of thin, flat, generally rectangular grids for carrying currents through the battery. The grids also serve as a substrate for supporting electrochemically active material or "paste" deposited thereon during manufacture to form battery plates. The paste typically comprises a mixture of lead oxide and a dilute acid solution, for example sulfuric acid. The paste composition is determined by power requirements, cost and the battery environment, as is known in the art.
It is also known that groupings of individual battery plates may be assembled, interleaved with separator material, and cast together to form plate stacks. The latter are then assembled in a container to form a final battery. A typical example would be an automobile battery of the 12-volt variety where, six individual stacks are placed into a plastic container having six compartments. Obviously, to permit current to follow throughout the battery, it is necessary to join the cast-on straps of one stack with the appropriate cast-on strap of the adjacent stack. Moreover, it is necessary to provide terminal electrodes which will extend through the casing to permit electrical contact with the vehicle's electrical system.
After the battery plates are fully assembled, they are formed, i.e. charged, and the battery is placed into service. An important characteristic of batteries is called "cold cranking power," i.e., the amount of power which can be generated per unit of time. Further references will be made to cold cranking power in later sections of this specification.
In more recent years, the grid has been formed in a screen-like pattern, having a matrix or a honeycomb pattern of alternating metal strips and open spaces. A porous chemical paste is applied to the grids and the lead frame members at the top, bottom and sides. The paste forms "paste pellets" within the open spaces of the grid. After the paste is cured, the electrodes are assembled into a battery casing, whereupon the housing is filled with aqueous electrolyte solution and sealed. In current systems, the lugs have been symmetrically offset from the centerline of the battery by amounts ranging from 1/2 inch to much larger amounts, and in many designs, the lugs of either the positive or negative battery plates are located near an upper corner of the plate. During assembly, it is necessary to couple together the positive lugs of the battery plates and to couple together the lugs of the negative plates. This is typically accomplished using cast-on lead straps formed by taking assembled battery plate stacks, inverting them, and dipping the lugs into molten lead contained within a mold. In the most common batteries, such cast-on straps have an upwardly extending end position, as will be explained more fully when the drawings are described later herein.
In U.S. Pat. No. 3,923,545, issued Dec. 2, 1975 to Margulies, et al., a "Grid Structure For High Rate Lead/Acid Battery" is described. In this device, the grid is constructed of more numerous vertical elements than horizontal elements and a collector tab is provided offset from the center of the top. A tapered portion is provided on the top frame element. In addition, at least two current-carrying grid elements are tapered in width and expand from the bottom of the plate toward the top thereof, the purpose of which is stated to be to maximize mechanical support while minimizing the resistive paths in regions of highest current density. In the background section of this patent, reference is made to an earlier patent wherein the tops of several "bars" nearest the terminal are thickened to provide ample current-carrying capacity in the region of the terminal lug.
Another earlier grid design is shown in U.S. Pat. No. 4,118,553, issued Oct. 3, 1978 to Buckenthal, et al. and entitled "Composite Battery Plate Grid And Method Of Manufacturing." This patent, owned by the assignee of the present invention, describes a composite grid including a plastic support having conductive members cast in a pattern extending generally radially from the collector lug, which is located near a corner of the grid. To attach the radial conductive members, the bottom of the lug extends laterally on both sides of the lug.
Still another grid construction in G.B. Pat. No. 2127614 issued Jan. 29, 1986 to Sonnenschein Accumulatoren and entitled "Electrode Grid for Storage Batteries" discloses a plurality of fine wires of a honeycomb structure with many equal elongated polygonal rhombs which is combined with a tapered herringbone central conductor. It is stated that a lower ohmic resistance is achieved within the grid plate and the amount of lead is reduced with this design.
Yet a different arrangement for grid elements is disclosed in Anderson, et al. U.S. Pat. No. 4,555,459, issued Nov. 26, 1985 and entitled "Battery Grids." The horizontal and angled wires defining this grid form a plurality of parallelogram spaces of substantially equal size, whereby paste pellets may be more uniformly created in the pasting operation. The lug is located on the top frame generally adjacent a corner, and in one embodiment, a tapered wire element is arranged vertically with the taper extending from bottom to top to divide the grid into two rectangular sections, each of which contains the parallelogram spaces.
A more recent grid arrangement with a center lug is shown in U.S. Pat. No. 5,308,719, issued May 3, 1994 to Mrotek et al., and entitled Lead Acid Batteries Containing Center Plates and High Performance Cast-On Straps. This patent, owned by the assignee of the present invention, discloses a grid construction in which the lug is formed with an expanded symmetrical base above the top of the frame member of the grid to provide a larger amount of conductive material on the sides of the lug. The purpose of the expanded base is for improving the collection of current collection of the lug. To maintain the weight of the grid at the same weight as a grid with a conventional lug, the conduction material on the grid wires is reduced by an offsetting amount.
While it is apparent from the foregoing description that numerous plate and grid designs have been developed in the lead acid battery art, improvements still need to be made in the design and manufacture of such batteries. One of the most significant improvements needed is a grid structure which reduces the weight associated with this type of energy source without deviating from the standard battery design with the conventional offset-from-the-center lug, but also a design which still can be used with unconventional lug positions on the top frame.
Furthermore, it is always advantageous to look for ways to maximize the efficiency of the grid conductors in transferring current from the sites where electrochemical reactions are occurring to the collector lugs as well as reducing the lead and, hence, the weight of the grid, on those conductors which are not the primary current conductors. Further, it is highly advantageous to use the optimum amount of lead in the grid where the current flow is the greatest.
Any lead acid battery which combines the various advantages discussed in the previous paragraphs would represent a substantial advance in the art.